Modulation Operation Check - Swann CUBIC ASTRO 102 BX Service Manual

I
OCUBIC
COMMUNICATIONS
S,.IIIII
DIVISION
ALC OPERATI ON
Disconnect the 50 ohm dummy load and substitute a standard 100W light
bulb (la ohms cold) as the load. Check the power output with the MIC GAIN
control fully clockwise on all bands. Power output should
be
40-70 watts if
the ALC circuit is operating properly.
With the antenna connector open (i.e., no load) , the total set current
in transmit should not exceed 12 amperes. If so, the ALC circuit is not
working properly.
NOTE: The purpose of the ALC is to limit power output and therefore PA
device dissipation to safe limits,
by
limiting collector current.
If there is too much ALC control, it will not be possible to ob-
tain full output into loads with SWRs less than 1.5:1. If there
is too little
ALe
control, then the PA transistors can draw ex-
cessive current, especially when the load is poorly matched, and
possibly go into oscillation, causing them to overheat and fail.
The temptation to increase the power output of the transmitter .ÈY
reducing ALC control must be resisted--to do so will almost
~­
antee the failure of expensive components in the PA.
MODULATION OPERATION
Check the two-tone signal on each band and in both USB and LSB modes
for crossover distortion, carrier leakage, and spurious outputs. None of
the above should be detectable in the output (see the "Servicing Hints" sec-
tion of this manual for examples and procedures for making the two-tone
test.)
Output should go to 100 W. PEP
without distortion being introduced
into the two-tone pattern.
NOTE: If a two-tone source is not available, a close examination of a
modulated output display using a high quality microphone andjor
monitoring the signal on another radio should show up any major
discrepancy, although not so accurately as does the two-tone
test.
I •
Two-tone and microphone modulation checks should
be
made wlth the spe-
ech processor off. Disconnect the two-tone generator and reconnect the mic-
rophone. The MIC LEVEL trimpot (R13S) on the audio board should
be
adjusted
for 100 W. PEP output with the MIC GAIN control set at la o'clock and speak-
ing into the microphone that is to be used with the unit, in anormal voice.
Note the average power reading on the wattmeter. Switch the SPE~H PROCES-
SOR on and note that the average power reading increases
by
approximately
20 watts. It should also be noticable on the oscilloscope display that the
peak to average modulation levels are less with the speech processor on than
when it is off.
With the VOX-PTT switch in the VOX position, adjust the VOX GAIN trim-
pot (Rl4O) on the audio board so that speaking into the microphone instantly
keys the unit. Adjust the ANTI TRIP trimpot (R149) on the audio board so
that audio from the speaker does not key the unit. Varying the VOX DELAY
trimpot (Rl44) on the audio board should vary the unkeying delay time from
o
to 5 seconds. There should be no loud popping noises in the speaker as
the unit keys or unkeys. Disconnect the microphone.
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